I removed the isolator and bought a continuous duty, heavy duty silver contact relay from http://texasindustrialelectric.com/relays.asp This way I'm always isolated from the alternator when the genny is on. I figured it takes about 50hp to get that 300 amps out of the Neihoff. I need that to the wheels, not my air conditioners. That power really makes a difference.

For what it's worth, 300 amps at 27 volts comes to 8.1 kw. The mechanical equivalent is less than 11 hp. The power wasted by the diodes at full output amounts to around 300 watts, and it's proportional to the power output. If the windings don't lose any more than the diodes, then you're looking at 600 watts at full blast, or less than one hp.

When unloaded, I don't think that these units take much to spin over. Mostly oil drag, I would guess.

I would have no trouble believing that one of these could soak up 12 hp., but that's about it. Even so, you're talking the better part of a gallon of fuel per hour while running full tilt.